The present invention relates to a display, and especially a color display, which comprises a fluorescent substance, and an organic electroluminescent light emitting device (which will hereinafter be often called an organic EL device for short) using an organic compound.
An organic EL device comprises a cathode and an anode with a fluorescent organic compound-containing thin film sandwiched between them. Upon electrons and holes injected into the thin film and recombined therein, excitons are generated. Upon deactivation of the excitons, the device emits light (fluorescence, and phosphorescence).
Such an organic EL device attracts attentions because they can achieve surface emission at a very high luminance ranging from a few hundred cd/m.sup.2 to 10,000 cd/m.sup.2 with a low driving voltage of the order of 10 volts.
A display constructed using such an organic EL device may potentially have various applications, and its application to color displays in particular is an important subject. For instance, a light emitting device may be applied to a color display in the following three ways. One possible approach uses a specific light emitting material for each of red, green, and blue pixels. One typical embodiment of this approach is known as set forth in SID 96 DIGEST. 185 14.2: Novel Transparent Organic Electroluminescent Devices G. Gu, V. Bulovic, P. E. Burrows, S. R. Forrest, M. E. Tompson. However, the color light emitting device (heterostructure organic light emitting device) described therein has a multilayer structure comprising light emitting layers (Red ETL, Green ETL, and Blue ETL) corresponding to R, G and B, respectively. If pixels are arranged in matrix array, then there is a limit to the mask used for pattern formation. This is because the organic EL device in film form is formed by a vacuum evaporation process. Further, the organic EL material is soluble in an organic solvent; difficulty is involved in photolithographic pattern formation. For this reason, there is a problem that no high precision display device is achieved.
Another possible approach employs an organic EL material emitting white light in combination with a color filter, thereby obtaining red, green, and blue light emissions. Never until now, however, is any organic EL material emitting white light by itself with high light emission efficiency known. White light may be obtained by allowing organic EL materials corresponding to red, green, and blue to give out light. However, this requires a complicated arrangement, and is not a technically realistic option. Here consider a supposed case where organic EL materials corresponding to red, green, and blue are allowed to emit light. Since the materials differ from each other in terms of light emission luminance and service life, the overall initial luminance is limited to that of the material having the lowest luminance. Also, these materials undergo color tone changes with time and make device design difficult, and so are not practical.
Yet another possible approach relies upon a specific organic EL material giving out blue light which is in turn converted to green light and red light through a color conversion layer comprising a fluorescent material. Making use of a single blue organic EL device alone, this approach is favorable because not only structural simplification but also cost reductions are achievable. Besides, if the color conversion layer is formed in a pattern-wise manner, images are then produced in full color. As typically referred to in JP-A 3-152897, however, a fluorescent material that can convert blue directly to red with high efficiency is virtually unheard-of because of unavailability of a fluorescent material showing a large Stokes' shift, such as one having a high absorption coefficient with respect to blue and a high fluorescence yield with respect to red. In view of the molecular orbit of an organic molecule, too, it is difficult to obtain a substance having such nature. Thus, a problem with this approach is that no high luminance display can be obtained because the overall light emission luminance should be regulated in conformity with red light of a low light emission luminance.
The conversion of blue light to red light may possibly be achieved by use of a simple mixture of a fluorescent material that absorbs blue light and gives out green light with a fluorescent material that absorbs green light and gives out red light. However, a color conversion material obtained by the simple mixing of fluorescent materials can not actually be used because fluorescence emission efficiency drops considerably due to interaction between the fluorescent materials.